Non specific building related illnesses or "sick building syndrome" are disorders of unknown cause that are estimated to affect 20-30 percent of workers in over one million buildings in the United States and result in loss of more than 10 billion dollars annually. Microbial contamination occurs in heating, ventilation, and air conditioning (HVAC) of most, if not all, modern office buildings and may result in specific building related outbreaks if heavy contamination occurs. There is considerable indirect evidence that microbial contamination of HVAC systems results in increased occurrence of non specific building related illnesses in office workers. In this study, ultraviolet lights will be installed in central HVAC systems of 4 office buildings which have sealed windows, mechanical HVAC systems, air conditioning, and humidification. In 3 consecutive blocks of 16 weeks the ultraviolet lights will be on for 4 consecutive weeks, and off for 12 consecutive weeks. Workers and data gatherers will be unaware of whether UV lights are on or off, making this a double blind study. On 2 occasions during each block, once with UV on and once with UV off, approximately 1,000 workers will be asked to rate their satisfaction with the indoor environment, report symptoms, occurrence of respiratory tract infections, sickness absence and complete typing tests. Extensive environmental measures will be taken in the same weeks including airborne and surface bacteria, fungal spores, and viable fungi will be measured at worksites and in the HVAC systems. Within-subject analysis will be used to compare occurrence of all study outcomes as well as microbial levels when UV lights are on or off. Funding has been obtained for the proposed study from the Medical Research Council of Canada (MRC). The funding awarded will cover all costs for basic environmental measures, recruitment, questionnaire, and typing test administration, as well as data analysis. Therefore, this request is for additional funding for endotoxin assays which is sought because endotoxins are an important potential pathogenetic mechanism which could account for substantial health effects resulting from microbial contamination of the central HVAC system. Airborne endotoxin concentrations would be reduced, and therefore the resultant health effects reduced, if GUV was successful in eradicating microbial contamination of central HVAC systems.